The invention relates to a transmission system comprising a transmitter and a receiver, the transmitter comprising first coding means for coding a first series of data symbols into a first series of signal elements and second coding means for coding a second series of data symbols into a second series of signal elements, the relation between the first series of data symbols and the first series of signal elements being different from the relation between the second series of data symbols and the second series of signal elements. The transmitter also comprises means for combining the two series of signal elements into a series of data pulses, and the transmission system comprises a channel for transmitting the data pulses to a receiver. At the input of the receiver a received signal will thereby consist of the first series of signal elements which has a first reference value at a first series of reference instants and the second series of signal elements which have a second reference value at a second series of reference instants. The receiver comprises sampling means for deriving a first or second sample value respectively, of the received signal at a first or second series of sampling instants respectively, and timing means for deriving the sampling instants from the data pulses. The invention likewise relates to such a receiver.
A transmission system as defined in the opening paragraph is known from the journal article "Q.P., An improved code for high density digital recording" by J. A. Bixby and R. A. Ketcham in IEEE Transactions on Magnetics, Vol. MAG-15, No. 6, November 1979. Transmission systems of this type may be used, for example, for data signal transmission via a transmission medium, for example, a cable link or radio link, or for reconstructing data signals which come from a magnetic tape or disc.
When data symbols are transmitted via a transmission medium or stored on a recording medium respectively, the symbols to be transmitted or recorded respectively, are coded by coding means into data pulses to be supplied to the transmission medium or recording medium, referred to as a "channel" hereinafter.
The form of the data pulses largely determines the properties of the signal transmitted through the channel. An important measure of the signal to be transmitted is the frequency spectrum which for many applications must meet various requirements.
A first desired property of the frequency spectrum is that the overall bandwidth of the signal remains limited because the cost of the necessary equipment rises according as the bandwidth of the signals to be processed is larger, for virtually all transmission mediums and recording mediums. A second desired property is that the DC component of the frequency spectrum should be zero. This property is important for channels which are incapable of passing DC components, which is the case in a number of frequently used channels. Examples of such channels are the public telephone network having a passband of 300 Hz to 3400 Hz, and magnetic recording channel for which the reading of magnetically stored information is based on the detection of flux changes in a reading head.
A method used for limiting the bandwidth of the signal is to split up a data stream to be transmitted or recorded into different groups of data symbols and generating data pulses which are each a sum of different signal elements, each signal element representing one of the symbols of the group of data symbols. Since a plurality of symbols are transmitted simultaneously, the duration of the signal elements may be shorter than when all data symbols are to be transmitted consecutively. As a result of this shorter duration of the signal elements the bandwidth of the data pulses may be smaller. However, the signal elements do have to be suitably selected to avoid mutual disturbance, so as to make it possible for the group of data symbols to be reconstructed from a received data pulse.
In the system known from above journal article signal elements are used which have a duration equal to twice the symbol interval T are used. The signal elements of the first coding means, depending on the binary value of the associated data symbol, adopt the values 1,0,-1,0 and -1,0,1,0 respectively, for a period of time 2.multidot.T. Depending on the binary value of the associated data symbol, the signal elements of the second coding means adopt the respective values 1,0,-1,0 and -1,0,1,0 for a period of time 2.multidot.T. The code leading to these signal elements is also known as Quadra-Phase code. However, that is not the only known code which consists of a sum of simultaneously transmitted signal elements.
The received signal is ideally sampled at sampling instants which coincide with reference instants at which one of the signal elements has an associated reference value. In the case of the Quadra-Phase code this reference value for both reference elements is equal to zero. In this manner sample values are obtained which according to the Quadra-Phase code only depend on the other signal element. These samples may then be used for the decision about the value of the associated data symbols.
If the sampling instants deviate from the reference instants, the obtained sample values will not only depend on a single signal element, but also on one (or more) other signal element(s), which will reduce the reliability of the decision made about the value of the associated data symbols.
In order to avoid this deviation, the prior-an system comprises means for deriving the sampling instants from the data pulses. As a result of the relatively small bandwidth of such codes, clock signal recovery for deriving the sampling instants from the received data symbols is rather difficult.
In the prior-art system a clock signal is derived from the received data pulses with the aid of a non-linear element having a fourth-power transfer chracteristic. A problem with such non-linear clock recovery systems is that the signal-to-noise ratio of the non-linearly processed input signal is always smaller than that of the input signal. Since noise in a clock signal manifests itself in the form of phase jitter, the recovered clock signal will contain rather much phase jitter when the input signal has a relatively low signal-to-noise ratio. The phase jitter results in the fact that the sampling instants are not always optimal.
The operation of the prior-art clock recovery system can be improved by including a narrow band phase-locked loop subsequent to the non-linear element, so that the signal-to-noise ratio of the clock signal may be improved. However, as a result of the small bandwidth of the phase-locked loop, the tracking rate of the clock recovery system will be relatively slow. If the transmission rate strongly fluctuates, such as, for example, on the magnetic recording channel or in a radio link between a fixed and a mobile station, it may happen that the narrow band phase-locked loop is incapable of keeping up with these fluctuations and so synchronization is lost.